The invention relates to an electrical multilayer contact, particularly for relays.
In the electrical switching, the following load conditions are distinguishable:
______________________________________ Type of Load Load Range Contact Stress ______________________________________ (a) dry circuits &lt;80 mV, &lt;10 mA no softening of thin layers of foreign material (b) low loads &lt;300 mV, &lt;100 mA contact area increases by softening (c) medium loads &lt;12 V, &lt;300 mA minor melting at the contact position (d) heavy loads &gt;12 V, &gt;300 mA burn-off of contact material by arc capacitive or inductive effects (e) a.c. voltage 6-12 V, &lt;5 A contact burn-off loads ______________________________________
A great number of contact materials is available to cope with these largely varying load conditions. For dry loads, for instance, alloys having a high gold content are suitable, such as AuCo 99/1, AuNi 97/3 or AuAg 90/10, since gold is very little corrosive and hardly affected by foreign layer formation. Moreover, since gold is relatively soft, a considerable contact surface is created even by small contact forces, thereby reducing the constriction resistance, which forms part of the contact resistance. Unalloyed gold is even somewhat too soft so that there may be a risk of contact sticking. This may be problematic specifically in relays without positive forced contact opening, such as reed relays. In this case, even mechanical vibrations as occur for instance during the cleaning process in an ultrasonic bath of circuit boards equipped with reed relays, may lead to a cold welding of normally closed contacts. As a counter-measure, the gold contact may be coated with a rhodium layer of a thickness in the Angstrom range which, due to its greater hardness, prevents contacts from sticking even when exposed to ultrasonic vibrations. While this measure increases the contact resistance by about 5%, the gold characteristic of the contact is essentially maintained.
In theory, relays may readily be provided with that contact material which is an optimum for any given load condition. However, disregarding those few cases in which relays with a specific contact material are required in large numbers, this is uneconomic, because too many different types would have to be manufactured in relatively small quantities.
For this reason, contacts for a wide load range have been developed. Such bi- or tri-metal contacts are disclosed in the book "Relais Lexikon" by H. Sauer, Deisenhofen 1975, page 49, FIG. 41. These relays comprise two or three layers wherein an about 0.2 mm thick layer of silver or an Ag-Ni alloy is disposed under an about 20 .mu.m thick gold layer. A basis is formed by a Cu-Ni alloy having an even higher burn-off resistance. Dry circuits as well as low, medium and heavy loads may be switched with contacts of this type. For a.c. voltage loads, however, the silver-nickel alloy is not particularly suited.
It is an object of the present invention to provide an electrical multilayer contact which is capable of a reliable switching over the entire range of the above-mentioned load conditions. As a further object, a multilayer contact of this type is to be provided which has a long useful life.